Fix recently introduced strong mode errors

pkg/analyzer/lib/src/task/dart.dart

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index 283d8204a65840fa3a1254ca94bf93bdb4d3a077..9437ad1630b04b05dc3f5113458f475f6d1964ae 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -1596,10 +1596,10 @@ class BuildLibraryElementTask extends SourceBasedAnalysisTask {
           context as InternalAnalysisContext;
       AnalysisCache analysisCache = internalContext.analysisCache;
       CacheEntry cacheEntry = internalContext.getCacheEntry(target);
-      libraryElement = analysisCache.getValue(target, LIBRARY_ELEMENT1);
+      libraryElement = analysisCache.getValue(target, LIBRARY_ELEMENT1) as LibraryElementImpl;
       if (libraryElement == null &&
           internalContext.aboutToComputeResult(cacheEntry, LIBRARY_ELEMENT1)) {
-        libraryElement = analysisCache.getValue(target, LIBRARY_ELEMENT1);
+        libraryElement = analysisCache.getValue(target, LIBRARY_ELEMENT1) as LibraryElementImpl;
       }
     }
     //